I think that you are most likely correct in thinking that Rachel ALEXANDER was born in NC and probably raised in Indiana. But if she married Larken Coffer in Missouri, her familymust have been in that state very early!
In the 1830 Missouri census index Larken COFFER is enumerated in St. Francois County on page 417. On that same page is Lawson ALEXANDER and on page 416 is Halem [sic?] ALEXANDER. I would suggest that these two ALEXANDER men are related to your Rachel. Do you know if this is the case? Would their ages indicate that they were brothers - or could one of them be her father?
There could be other ALEXANDERS in that county at that time also - who haven't made it into the index.